The invention relates to a device for heating and drying parts with cellulose or plastic based hygroscopic electric insulation, such as contained in transformers, condensers, electric lead through terminals, etc.
To avoid failures, power transformers in substations in the electrical energy supply grid require preventive preparations of their hygroscopic insulations. This is also true after repairs or if malfunctions have occurred, if after the apparatuses have been opened the hygroscopic insulations of the transformers have absorbed moisture and gas from the ambient air, reducing their insulating properties. Deposits of aging products also adversely affect the insulating capability.
In maintenance or in the event of minor malfunctions, the active part of the transformer can stay in the housing and be re-prepared locally, that is, in the substation, for further use. Several methods already exist for accomplishing this.
Preparation by means of the insulating oil is a widely used method. In it, the operating oil is drawn from the transformer by a recirculating process, carried in a closed loop through a vacuum preparation system, demoistened and degassed in the process, and returned to the transformer. By the contact of the dried oil with the insulation, a withdrawal of moisture from the insulation thus takes place indirectly via the preparation system.
In a further process, known as the xe2x80x9coil spray processxe2x80x9d, oil is for the most part drained out of the transformer, so that the insulations can be exposed to the vacuum. By spraying insulating oil into the interior of the transformer, the transformer is heated, and the evaporating moisture is aspirated by a vacuum pump connected to it.
Combinations of the aforementioned processes also exist in conjunction with recirculated hot air or partial direct heating of the windings by passing an electrical current through them.
All the above methods, however, have the disadvantage that the temperature range for the drying is very severely limited by the presence and use of the insulating oil. The impregnation also severely impairs the diffusion of water vapor. The preparation process is therefore extremely time-consuming. A further disadvantage is that the insulation is inadequately heated at places that are unfavorable in terms of flow and especially in poorly accessible conduits is not cleaned of the deposits.
In the event of major defects, the transformer must therefore be transported away from the distributor station to a repair facility, where it is broken down, repaired, and then re-prepared as described above.
Nowadays the insulations of power transformers, and especially the windings assembled to make the active part, are typically dried, upon initial production, at the producing factory by so-called vapor-phase processes in accordance with German Patent Disclosures DE 198 26 682 A1, DE 196 37 313 A1, or German Patent DE 44 46 204 C1. In these processes, the active part with the insulator parts contained in it is heated in a large vacuum autoclave with the solvent vapor, produced in an evaporator, and in the process subjected to intensive cleaning.
These methods are extremely effective, but they are poorly suited to on-site preparation of transformers that are already in operation, since the large cross sections toward the transformer that are required for delivering the heat transfer medium in vapor form cannot be achieved in a portable drying device.
With this as the point of departure, it is the object of the invention to overcome the aforementioned disadvantages and to prepare transformers, especially on-site, and in the process subject them to an intensive washing process. In this process, the absorbed oil should preferably also be precipitated out.
Since with the invention as many of the advantages named in the three references cited above are to be attained or preserved, the teaching in these references is hereby explicitly fully incorporated by reference.
According to the invention, a heat exchanger heats the heating fluid in such a way that it stays in the liquid phase. The evaporation does not take place until on or in the vessel, for example via expansion valves, or in an expansion container disposed directly on the vessel, via the pressure drop that takes place there, or via a separate evaporator.
The device can be either stationary or mobile, in which case the transformer housing of the transformer to be heated can be used as the vessel. It is favorable if the connections between the heat exchanger that heats the heating fluid and the expansion valves and/or between an outlet valve from the transformer housing and a feed pump of the system and/or between the transformer housing and one or more blocking valves are disconnectable. Then the system can easily be transported to its applicable locations of use and connected there.
If a removable solvent container is used for the heating fluid, then used heating fluid, whose properties have been impaired by transformer oil, can be removed directly and prepared elsewhere, and new fluid can very easily be supplied to the transformer.
To improve the efficiency, the transformer housing can be heated in insulated fashion or externally by portable insulating means, such as insulating plates, insulating mats, wall elements, single- or double-walled supporting air tents, or by portable supplementary heaters; for mobile operation, air heaters are an attractive choice.
Instead of expansion valves, which expand the heating fluid into the transformer housing and cause it to evaporate there, an expansion container with a short line to the transformer housing can also be used to evaporate the heating fluid. This expansion container can have a return line for the unevaporated heating fluid. The critical path for the evaporated heating fluid thus once again remains short, as desired.
The expansion container can also be disposed on the transformer housing. Once again, a short line to the transformer housing for the evaporated heating fluid is important. If an inlet line to a condenser is provided, then once the heating is concluded, better distillation of the solvent portion off from the mixture is possible.
If the expansion container has a permeable double jacket, then the heated heating fluid, to cover the heat losses, can flow through the expansion container before being evaporated.
In one version of the invention, it is additionally possible, for separating the already-concentrated heating fluid flowing out of the expansion container, to provide a thin-film evaporator. If the thin-film evaporator has an inlet line to a condenser, then the vapor that occurs at reduced pressure can be delivered directly to the condenser.
In a further version, a lightweight, compact thin-film evaporator, such as a plate evaporator, can be secured directly to the transformer housing. Once again, short paths for the vapor are then attained.
In a further embodiment of the invention, it is provided that the already-concentrated proportion flowing out of the expansion container is delivered through a valve and a pressure barrier by gravity to a downstream thin-film evaporator for filtering off the rest of the solvent, and the vapor occurring when the pressure has been lowered is delivered through the valve directly to the condenser. In still another embodiment of the invention, it is provided that the pressure barrier is formed by a feed pump with a valve circuit, and the post-evaporator is set up inside the mobile system. It is attained as a result that in the preparatory phase the evaporator need not be secured to the transformer housing, which considerably reduces the manual effort required.
If the transformer is located so low that there is no space to hold the heating fluid flowing out of the vessel, then a feed pump, designed for low inlet heights, which is preferably embodied as a liquid ring pump or as a positive displacement pump, and an intermediate container, disposed at a higher level are used, with a downstream centrifugal ring pump for the further pumping of the heating fluid into the heat exchanger and/or into the evaporators. The intermediate container thus makes an adequate inlet height for the centrifugal pump possible.
For more-effective cleaning of the solvent at the end of the heating and after the conclusion of the pressure reduction, a thin-film evaporator, with an inlet line to a condenser and with an outlet line to the solvent container, can be provided.
Further objects, advantages, characteristics and possible uses of the present invention will become apparent from the ensuing description of the exemplary embodiments in conjunction with the drawings. All the characteristics described and/or shown in the drawings, on their own or in arbitrary useful combination, form the subject of the present invention, independently of how they are combined in the claims or of the claims dependency.